Surface the sidecar duration on the first Opus chunk instead of gating it on the first decoded buffers; C# locks UI Duration on chunk 1, and async WebCodecs decode left it at 0 — killing seek and the duration-gated visualizer.
Seek now trims the lead-in so playback lands at the requested time, not the page start; decoder drain polls decodeQueueSize (bounded) instead of a single timeout. Minor cleanups.